Synthesis of metatungstate pillared layered double hydroxides with variable layer composition. Effect of the Mg:Al ratio on the microporous structure

A series of layered double hydroxides (LDHs) pillared with the Keggin ion H₂W₁₂O₄₀⁶⁻ has been synthesized with final Mg:Al ratios of 1.75:1, 2.31:1 and 3.51:1 by ion exchange reaction of the LDH-hydroxide and LDH-adipate precursors. In each case, the pillared product was characterized by specific BET N₂ surface areas of more than 110 m² g⁻¹ and micropore volumes in excess of 0.026 ml g⁻¹. The micropore size distribution plots obtained from the argon adsorption isotherms indicate that the micropore diameters become smaller as the surface charge density of the LDH increases. A study was also performed on each of the LDH precursors in order to determine any problematic steps in the overall synthesis. Significant differences were noticed in the POXRD patterns of the LDH-adipate precursor; these are believed to be due to differences in the orientation and degree of order of the adipate anions in the gallery space of the LDH. It was found that adipate anions orientated with the long axis perpendicular to the inorganic layers were easier to exchange than those where the long axis was aligned parallel to the inorganic layers.